Intake-air cooling device for engine and method for cooling engine

ABSTRACT

A first cooling water circuit is configured such that the cooling water that has come out from the engine passes through the first intake-air cooling device and returns to the engine again. A second cooling water circuit is configured such that the cooling water that has come out from the engine passes through the second intake-air cooling device and returns to the engine again. A heat exchanger is configured to perform heat exchange between the cooling water that has come out from the engine and flowing to the first intake-air cooling device and the cooling water that has come out from the second intake-air cooling device and that is flowing to the engine.

TECHNICAL FIELD

The invention relates to an intake-air cooling device provided on a supercharged engine.

BACKGROUND ART

An engine provided with a supercharger that supercharges intake air by using exhaust gas from the engine is generally used. As the intake air is supercharged by the supercharger, temperature of the intake air is increased to a high temperature. In a case where an EGR system in which the exhaust gas is recirculated to the intake air side is provided, the temperature of the intake air may be increased further to a higher temperature. If the temperature of the intake air is high, there is a risk that a fuel consumption efficiency is reduced.

In order to prevent the fuel consumption efficiency from being reduced, a cooling device is provided for decreasing the temperature of the supercharged intake air. In the cooling device, for example, cooling water for an engine is allowed to flow into an intake air channel, and the temperature of the intake air is decreased with the cooling water.

On the other hand, because the temperature of the cooling water for the engine is controlled so as to achieve an optimal temperature for operation of the engine, sufficiently large air-water temperature difference between the intake air and the cooling water cannot be achieved, and the decrease in the temperature of the intake air is limited.

In order to solve the problem, in US2008/0066697A, a cooling device for an internal combustion engine including a second radiator on a second cooling water circuit branched off from a part of a cooling water circuit of the engine is described. With this cooling device, the temperature of the intake air to be introduced into the engine is decreased with cooling water whose temperature has been decreased by the second radiator.

SUMMARY OF INVENTION

When a cooling water temperature is decreased in order to decrease a temperature of intake air, the low-temperature cooling water flows into an engine. In the case in which the low-temperature cooling water is introduced into the engine, especially when the engine is being warmed up, the cooling water temperature in the engine is decreased, and the warming up of the engine is delayed. If the warming up of the engine is delayed, there is a problem in that a fuel consumption performance of the engine is deteriorated.

This invention has been designed in consideration of the problems described above, and an object thereof is to provided an intake-air cooling device for an engine that is capable of improving the delay in the warming up of the engine while including a cooling device cooling intake air for the engine.

According to one aspect of this invention, an intake-air cooling device for an engine that is provided on the engine including a cooling water circuit through which cooling water for the engine flows and a supercharger that supercharges intake air for the engine, comprises a first intake-air cooling device, a second intake-air cooling device, and a heat exchanger. The cooling water circuit has a first cooling water circuit and a second cooling water circuit. The first intake-air cooling device is configured such that the intake air is cooled with the cooling water in the first cooling water circuit, and the second intake-air cooling device is configured such that, with the cooling water in the second cooling water circuit, the intake air that has been cooled by the first intake-air cooling device is further cooled. The first cooling water circuit is configured such that the cooling water that has come out from the engine passes through the first intake-air cooling device and returns to the engine again; the second cooling water circuit is configured such that the cooling water that has come out from the engine passes through the second intake-air cooling device and returns to the engine again. The heat exchanger is configured to perform heat exchange between the cooling water that has come out from the engine and flowing to the first intake-air cooling device and the cooling water that has come out from the second intake-air cooling device and that is flowing to the engine.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory diagram of a cooling device according to a first embodiment of the present invention showing an engine at the center thereof.

FIG. 2 is an explanatory diagram of a cooling device according to a second embodiment of the present invention showing an engine at the center thereof.

FIG. 3 is a flowchart of processing executed by a controller according to the second embodiment of the present invention.

FIG. 4 is an explanatory diagram of a cooling device according to a third embodiment of the present invention showing an engine at the center thereof.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is an explanatory diagram of a cooling device 1 according to a first embodiment of the present invention showing an engine 10 at the center thereof.

The cooling device 1 according to the first embodiment includes a supercharger (turbine) 18 and the engine 10 that is mounted on, for example, a vehicle so as to serve as a driving source of the vehicle, and uses cooling water (coolant) to suitably decrease a temperature of supercharged intake air.

In FIG. 1, thick arrows show a high-temperature-side cooling water circuit 31, narrow arrows show a low-temperature-side cooling water circuit 32, dotted lines show flows of exhaust gas in exhaust gas pipes 16, and one-dot chain lines show flows of intake air in intake-air pipes 14.

The cooling device 1 includes the engine 10 and a cooling water circuit 30 through which cooling water for the engine 10 flows.

Inside the engine 10, a cooling-water flow path 11 through which the cooling water flows is formed. The cooling-water flow path 11 is in communication with the cooling water circuit 30. The cooling-water flow path 11 includes a water pump (W/P) 12 and a thermostat (T/S) 13.

The cooling water is circulated through the cooling-water flow path 11 and the cooling water circuit 30 by the water pump 12. When the temperature of the cooling water is low, a radiator 41 is bypassed by the thermostat 13, and when the temperature of the cooling water is high, the cooling water temperature is decreased by guiding the cooling water to the radiator 41.

The engine 10 is in communication with the intake-air pipes 14 and the exhaust gas pipes 16. The intake air is supercharged to the intake-air pipes 14 by the turbine 18. The temperature of the supercharged intake air is decreased by a high-temperature side intercooler (first intake-air cooling device) 71 and a low-temperature side intercooler (second intake-air cooling device) 72, and the intake air whose temperature has been decreased is sent to the engine 10. The exhaust gas pipes 16 allow exhaust gas from the engine 10 to be discharged through the turbine 18. The exhaust gas rotates the turbine 18, and the rotation of the turbine 18 makes the intake air in the intake-air pipes 14 to be supercharged.

The engine 10 includes a fan 19. As the fan 19 sends wind to the radiator 41 and a sub-radiator 42, the cooling at the radiator 41 and the sub-radiator 42 is facilitated.

An EGR circuit 20 is branched off from the exhaust gas pipes 16. The EGR circuit 20 forms an exhaust gas recirculation device (EGR) that recirculates a part of the exhaust gas into the intake air. The EGR circuit 20 includes a first EGR cooler (first exhaust-gas cooling device) 21 at a high-temperature side and a second EGR cooler (second exhaust-gas cooling device) 22 at the same high-temperature side, which is provided downstream of the first EGR cooler 21, and communicates with the intake-air pipes 14 through an EGR valve 23.

By returning a part of the exhaust gas into the intake air through the EGR circuit 20, it is possible to decrease oxygen concentration in combustion chambers of the engine 10 to decrease the combustion temperature, thereby suppressing generation of oxides, such as NOx etc. Because the efficiency increases with decrease in the temperature of the exhaust gas recirculated, in order to decrease the exhaust gas temperature, the first EGR cooler 21 at the high-temperature side and the second EGR cooler 22 at the same high-temperature side, which is provided downstream of the first EGR cooler 21, are provided.

In the intake-air pipes 14, the cooling water respectively flows through the high-temperature side intercooler 71 and the low-temperature side intercooler 72, and the temperature of the intake air supercharged by the turbine 18 is decreased. Because the temperature of the intake air may become high and the temperature difference with the cooling water is large, the temperature of the intake air is decreased at two stages, namely, the high-temperature side intercooler 71 and the low-temperature side intercooler 72. The temperature of the intake air whose temperature has been decreased by the high-temperature side intercooler 71 is further decreased by the low-temperature side intercooler 72. In the intake-air pipes 14, the EGR valve 23 is provided downstream of the high-temperature side intercooler 71 and the low-temperature side intercooler 72. The EGR valve 23 controls the amount of the exhaust gas recirculated in the intake-air pipes 14 through the EGR circuit 20.

The cooling water circuit 30 includes the high-temperature-side cooling water circuit (first cooling water circuit) 31 and the low-temperature-side cooling water circuit (second cooling water circuit) 32.

The high-temperature-side cooling water circuit 31 is formed of a cooling water circuit that extends through, in addition to the cooling-water flow path 11 of the engine 10, the radiator (first radiator) 41, the first EGR cooler 21 at the high-temperature side, and the second EGR cooler 22 at the same high-temperature side, which is provided downstream of the first EGR cooler 21.

In the high-temperature-side cooling water circuit 31, the cooling water sent out from the water pump 12 of the engine 10 circulates through the cooling-water flow path 11 of the engine, passes through the radiator 41, and returns to the cooling-water flow path of the engine 10 again. A part of the cooling water sent out from the water pump 12, after coming out from the engine 10, passes through the first EGR cooler 21 at the high-temperature side and the second EGR cooler 22 at the same high-temperature side, which is provided downstream of the first EGR cooler 21, and returns to the cooling-water flow path 11 of the engine 10 again. A part of the cooling water sent out from the water pump 12 comes out from the engine 10, passes through a heat exchanger 76 and the high-temperature side intercooler 71, and returns to the cooling-water flow path 11 of the engine 10 again.

The low-temperature-side cooling water circuit 32 is formed of a cooling water circuit passing through a thermostat 44, the sub-radiator (second radiator) 42, the low-temperature side intercooler 72, and the heat exchanger 76.

The cooling water sent out from the water pump of the engine 10 comes out from the engine 10, passes through the thermostat 44 and the sub-radiator 42, and is sent to the low-temperature side intercooler 72. After coming out from the low-temperature side intercooler 72, the cooling water returns to the cooling-water flow path 11 of the engine 10 again through the heat exchanger 76. When the temperature of the cooling water in the low-temperature-side cooling water circuit 32 is low, the thermostat 44 makes the sub-radiator 42 to be bypassed, thereby preventing a further decrease in the cooling water temperature.

In this way, the low-temperature-side cooling water circuit 32 is formed such that, by passing through the sub-radiator 42, the cooling water flowing therethrough has the temperature lower than that of the cooling water flowing through the high-temperature-side cooling water circuit 31.

In the heat exchanger 76, heat exchange is performed between the cooling water that has come out from the engine 10 in the high-temperature-side cooling water circuit 31 and the cooling water that has come out from the low-temperature side intercooler 72 in the low-temperature-side cooling water circuit 32. The heat exchanger 76 has, for example, a double-pipe structure, and performs the heat exchange by being configured such that the cooling water in the high-temperature-side cooling water circuit 31 and the cooling water in the low-temperature-side cooling water circuit 32 flow in the opposite directions from each other.

An operation of the first embodiment of the present invention configured as described above will be described.

When the engine 10 is started in a state in which the temperatures of the engine 10 and the cooling water are low (cold start), such as when a long period of time has passed since the engine 10 was stopped, because sliding resistance is high and efficiency of catalyst is also low in the engine 10, the operation efficiency of the engine 10 is low, and the fuel consumption performance is deteriorated, thereby causing the amount of regulated pollutants contained in the exhaust gas to be increased. Thus, when the engine 10 is cold-started, the warming up of the engine 10 needs to be performed as quickly as possible.

With the above-mentioned configuration, the cooling device 1 according to the first embodiment of the present invention is thus operated in a manner described below.

In the engine 10, the cooling water is circulated through the cooling-water flow path 11 by the water pump 12. At this time, when the cooling water temperature is low, the thermostat 13 is switched such that the radiator 41 is bypassed. With such a configuration, as the engine 10 is operated, the cooling water is heated and the cooling water temperature is increased.

A part of the cooling water flowing through the cooling-water flow path 11 flows into the high-temperature-side cooling water circuit 31, passes through the first EGR cooler 21 at the high-temperature side and the second EGR cooler 22 at the same high-temperature-side, which is provided downstream of the first EGR cooler 21, and returns to the cooling-water flow path 11. A part of the cooling water in the high-temperature-side cooling water circuit 31 passes through the high-temperature side intercooler 71, and returns to the cooling-water flow path 11.

As described above, the high-temperature-side cooling water circuit 31 is configured such that the temperature of the cooling water is prevented from being decreased by bringing the cooling water into contact with the high-temperature exhaust gas. In the high-temperature side intercooler 71, the decrease in the temperature of the cooling water is suppressed by bringing the cooling water into contact with the supercharged high-temperature intake air. With such a configuration, even when the engine 10 is cold-started, delay in the warming up of the engine 10 is improved by the flow of the cooling water at relatively high temperature.

A part of the cooling water flowing through the cooling-water flow path 11 flows into the low-temperature-side cooling water circuit 32, passes through the sub-radiator 42, the low-temperature side intercooler 72 and the heat exchanger 76, and returns to the cooling-water flow path 11. The temperature of the cooling water in the low-temperature-side cooling water circuit 32 is decreased via the heat exchange with the outside air performed in the sub-radiator 42. The cooling water whose temperature has been decreased undergoes the heat exchange with the supercharged intake air in the low-temperature side intercooler 72, and thereby, the temperature of the intake air is decreased and the cooling water temperature is increased. After coming out from the low-temperature side intercooler 72, the temperature of the cooling water is further increased by undergoing heat exchange at the heat exchanger 76 with the cooling water in the high-temperature-side cooling water circuit 31. The cooling water whose temperature has been increased returns to the engine 10 again. Although the temperature of the cooling water in the high-temperature-side cooling water circuit 31 is slightly decreased by undergoing the heat exchange at the heat exchanger 76 with the cooling water in the low-temperature-side cooling water circuit 32, the temperature of the cooling water in the high-temperature-side cooling water circuit 31 is increased by undergoing heat exchange at the high-temperature side intercooler 71 with the supercharged high-temperature intake air, and the cooling water returns to the engine 10 again.

As described above, in the low-temperature-side cooling water circuit 32, the temperature of the supercharged intake air is decreased at the low-temperature side intercooler 72 with the cooling water whose cooling water temperature has been decreased at the sub-radiator 42.

In this configuration, the cooling water whose temperature has been decreased passes through the low-temperature side intercooler 72 and the heat exchanger 76 to increase its temperature again, and the cooling water whose temperature has been increased returns to the engine 10 again. The cooling water returns to the engine 10 through the circuit where the high-temperature-side cooling water circuit 31 and the low-temperature-side cooling water circuit 32 are joined. At this time, the cooling water in the high-temperature-side cooling water circuit 31 and the cooling water in the low-temperature-side cooling water circuit 32 are mixed. With such a configuration, in the low-temperature-side cooling water circuit 32, because the temperature of the cooling water returning to the engine 10 is prevented from being decreased, the delay in the warming up of the engine 10 is improved.

As described above, the first embodiment of the present invention is configured such that the cooling water that has come out from the engine 10 in the high-temperature-side cooling water circuit 31 and the cooling water that has come out from the low-temperature side intercooler 72 in the low-temperature-side cooling water circuit 32 undergo the heat exchange at the heat exchanger 76.

Especially, in the low-temperature-side cooling water circuit 32, because the temperature of the cooling water is decreased at the sub-radiator 42, it is possible to decrease the temperature of the supercharged intake air and to suppress generation of NOx while improving the operation efficiency of the engine.

Because the temperature of the cooling water returning to the engine 10 in the low-temperature-side cooling water circuit 32 is increased by passing through the heat exchanger 76, the delay in the warming up of the engine 10 is improved. Because the temperature of the cooling water flowing through the high-temperature side intercooler 71 in the high-temperature-side cooling water circuit 31 is decreased by passing through the heat exchanger 76, it is possible to decrease the temperature of the intake air at the high-temperature side intercooler 71. Because the cooling water that has come out from the high-temperature side intercooler 71 is combined with the cooling water flowing just before the engine 10 in the low-temperature-side cooling water circuit 32, and the combined cooling water then flows into the engine 10, the temperature of the cooling water flowing through the engine 10 is increased, thereby improving the delay in the warming up of the engine 10.

With such a configuration, even when the engine 10 is cold-started, the delay in the warming up of the engine 10 is improved.

Next, a second embodiment of the present invention will be described.

FIG. 2 is an explanatory diagram of the cooling device 1 according to the second embodiment of the present invention showing the engine 10 at the center thereof. Components that are the same as those in the first embodiment are assigned the same reference signs, and a description thereof shall be omitted.

In the second embodiment, in the low-temperature-side cooling water circuit 32, a valve 85 is provided at the inlet side of the heat exchanger 76, and a bypass channel 86 bypassing the heat exchanger 76 is provided. By opening/closing the valve 85, the flow of the cooling water flowing through the heat exchanger 76 in the low-temperature-side cooling water circuit 32 is controlled.

In the second embodiment, the cooling device 1 includes a first water temperature gauge 81 that detects the water temperature of the cooling water flowing into the heat exchanger 76 from the high-temperature-side cooling water circuit 31 and a second water temperature gauge 82 that detects the water temperature of the cooling water flowing into the heat exchanger 76 from the low-temperature-side cooling water circuit 32. A controller 60 is provided so as to perform opening/closing control of the valve 85 on the basis of a water temperature TwH in the high-temperature-side cooling water circuit 31 detected by the first water temperature gauge 81 and a water temperature TwL in the low-temperature-side cooling water circuit 32 detected by the second water temperature gauge 82.

Next, an operation of the thus-configured cooling device 1 according to the second embodiment will be described.

FIG. 3 is a flowchart of a control of a cooling water circuit executed by the controller 60 according to the second embodiment of the present invention.

The flowchart shown in FIG. 3 is executed by the controller 60 when the engine 10 is started.

First, the controller 60 checks whether the cooling water temperature has reached a valve opening temperature of the thermostat 44 in the low-temperature-side cooling water circuit 32 (Step S10). When the cooling water temperature has not reached the valve opening temperature of the thermostat 44, because it is determined that the cooling water temperature is low, it is possible to determine malfunction of the first water temperature gauge 81 and the second water temperature gauge 82. It may be possible to check whether the thermostat 44 is opened, instead of checking the cooling water temperature.

Next, the controller 60 detects the water temperature TwH at the inlet side of the heat exchanger 76 in the high-temperature-side cooling water circuit 31 through the first water temperature gauge 81. The controller 60 detects the water temperature TwL at the outlet side of the heat exchanger 76 in the low-temperature-side cooling water circuit 32 through the second water temperature gauge 82. Then, the controller 60 determines if the water temperature TwL is lower than the water temperature TwH (Step S20).

When the water temperature TwL is determined to be lower than the water temperature TwH, the process advances to Step S30, and the controller 60 performs a control so as to open the valve 85. By performing the control in this way, the cooling water in the low-temperature-side cooling water circuit 32 is made to pass through the heat exchanger 76, and thereby, at the heat exchanger 76, the cooling water in the low-temperature-side cooling water circuit 32 and the cooling water in the high-temperature-side cooling water circuit 31 undergo the heat exchange. Thereafter, the process advances to Step S40.

In Step S40, the controller 60 determines whether the water temperature TwL is higher than the water temperature TwH. When the water temperature TwL is lower than the water temperature TwH, the controller 60 repeats Step S40 and waits. In this case, the valve 85 is left open, and at the heat exchanger 76, the cooling water in the low-temperature-side cooling water circuit 32 and the cooling water in the high-temperature-side cooling water circuit 31 undergo the heat exchange.

When the water temperature TwL is determined to be higher than the water temperature TwH, the process advances to Step S50, and the controller 60 performs the control so as to close the valve 85. By performing the control in this way, the cooling water in the low-temperature-side cooling water circuit 32 passes through the bypass channel 86 without passing through the heat exchanger 76, and does not undergo the heat exchange with the cooling water flowing through the heat exchanger 76 in the high-temperature-side cooling water circuit 31. Thereafter, the process advances to Step S60.

In Step S20, when the water temperature TwL is determined to be equal to or higher than the water temperature TwH, the processes of Steps S30 and S40 are not performed, in other words, the process advances to Step S50 without opening the valve 85, and the controller 60 performs the control so as to close the valve 85.

In Step S60, the controller 60 determines whether the operation of the engine 10 is stopped. If the engine 10 is under operation, the process returns to Step S20, and the processes are repeated. If the operation of the engine 10 is stopped, the process shown in this flowchart is terminated.

As described above, with the cooling device 1 of the second embodiment, the controller 60 determines whether the valve 85 is to be opened to perform the heat exchange at the heat exchanger 76, on the basis of the water temperature TwL in the low-temperature-side cooling water circuit 32 and the water temperature TwH in the high-temperature-side cooling water circuit 31.

In other words, when the temperature TwL of the cooling water flowing into the heat exchanger 76 in the low-temperature-side cooling water circuit 32 is lower than the temperature TwH of the cooling water in the high-temperature-side cooling water circuit 31, the heat exchange is performed at the heat exchanger 76. With such a configuration, because it is possible to increase the cooling water temperature in the low-temperature-side cooling water circuit 32, the temperature of the cooling water flowing into the engine 10 is not decreased, and therefore, the delay in the warming up of the engine is improved. By performing the heat exchange at the heat exchanger 76, it is possible to decrease the cooling water temperature in the high-temperature-side cooling water circuit 31, and to decrease the temperature of the supercharged intake air at the high-temperature side intercooler 71.

On the other hand, when the temperature TwH of the cooling water in the high-temperature-side cooling water circuit 31 is lower than the temperature TwL of the cooling water flowing into the heat exchanger 76 in the low-temperature-side cooling water circuit 32, the heat exchange is not performed at the heat exchanger 76. With such a configuration, because the heat exchange is not performed with the cooling water whose temperature is lower than that of the cooling water in the low-temperature-side cooling water circuit 32, the temperature of the cooling water flowing into the engine 10 is not decreased, and the delay in the warming up of the engine is improved. Because the heat exchange is not performed with the cooling water in the high-temperature-side cooling water circuit 31 whose cooling water temperature is a high temperature, it is possible to further decrease the temperature of the supercharged intake air at the high-temperature side intercooler 71.

Next, a third embodiment of the present invention will be described.

FIG. 4 is an explanatory diagram of the cooling device 1 according to the third embodiment of the present invention showing the engine 10 at the center thereof. Components that are the same as those in the first and second embodiments are assigned the same reference signs, and a description thereof shall be omitted.

In the third embodiment, a low-temperature-side third EGR cooler 24 is provided instead of the high-temperature-side second EGR cooler 22 in the first or second embodiment, and the cooling device 1 is configured such that the cooling water in the low-temperature-side cooling water circuit 32 flows into the low-temperature-side third EGR cooler 24. The cooling device 1 includes a second heat exchanger 46 that performs the heat exchange between the cooling water in the high-temperature-side cooling water circuit 31 that flows into the high-temperature-side first EGR cooler 21 provided in the EGR circuit 20 and the cooling water that has come out from the low-temperature-side third EGR cooler 24 in the low-temperature-side cooling water circuit 32. As described above, because the efficiency increases with decrease in the temperature of the intake air of the engine 10, in order to decrease the temperature of the exhaust gas to be recirculated by EGR, it is configured such that the low-temperature cooling water that has come out from the sub-radiator 42 flows into the low-temperature-side third EGR cooler 24.

In the high-temperature-side cooling water circuit 31, a part of the cooling water sent out from the water pump 12 of the engine 10 comes out from the engine 10, passes through the second heat exchanger 46 and the high-temperature-side first EGR cooler 21, and returns to the cooling-water flow path 11 of the engine 10 again.

In the low-temperature-side cooling water circuit 32, a part of the cooling water that has come out from the sub-radiator 42 is sent to the low-temperature-side third EGR cooler 24. The cooling water that has come out from the low-temperature-side third EGR cooler 24 passes through the second heat exchanger 46 and returns to the cooling-water flow path 11 of the engine 10 again.

In the second heat exchanger 46, the heat exchange is performed between the cooling water that has come out from the engine 10 in the high-temperature-side cooling water circuit 31 and the cooling water that has come out from the low-temperature-side third EGR cooler 24 in the low-temperature-side cooling water circuit 32. Similarly to the heat exchanger 76, the second heat exchanger 46 has, for example, a double-pipe structure, and performs the heat exchange by being configured such that the cooling water in the high-temperature-side cooling water circuit 31 and the cooling water in the low-temperature-side cooling water circuit 32 flow in the opposite directions from each other.

As described above, in the third embodiment, in order to cool the exhaust gas that is recirculated by the EGR, although it is common with the above-mentioned first and second examples to provide the high-temperature-side first EGR cooler 21 and the low-temperature-side third EGR cooler 24 on the intake-air pipes 14, the low-temperature-side third EGR cooler 24 is configured such that the exhaust gas temperature is decreased with the cooling water whose cooling water temperature has been decreased in the sub-radiator 42.

Because the high-temperature-side cooling water circuit 31 is configured such that the temperature of the cooling water is prevented from being decreased by bringing the cooling water into contact with the high-temperature exhaust gas, even when the engine 10 is cold-started, delay in the warming up of the engine 10 is improved by the flow of the cooling water at relatively high temperature.

In the low-temperature-side cooling water circuit 32, the cooling water whose temperature has been decreased by the sub-radiator 42 passes through the low-temperature-side third EGR cooler 24 and the second heat exchanger 46, and returns to the cooling-water flow path 11. By performing the heat exchange between the cooling water in the low-temperature-side cooling water circuit 32 and the exhaust gas at the low-temperature-side third EGR cooler 24, the exhaust gas temperature is decreased and the cooling water temperature is increased. The cooling water that has come out from the low-temperature-side third EGR cooler 24 undergoes the heat exchange at the second heat exchanger 46 with the cooling water in the high-temperature-side cooling water circuit 31, causing its temperature to be further increased. The cooling water whose temperature has been increased returns to the engine 10 again.

The cooling water in the high-temperature-side cooling water circuit 31 and the cooling water in the low-temperature-side cooling water circuit 32 are combined at a point just upstream of the engine 10, and the combined cooling water returns to the engine 10. With such a configuration, in the low-temperature-side cooling water circuit 32, the temperature of the cooling water returning to the engine 10 is not decreased, and therefore, the delay in the warming up of the engine 10 is improved.

Also in the third embodiment, the controller 60 may determine whether the second heat exchanger 46 is to be bypassed on the basis of the water temperature in the low-temperature-side cooling water circuit 32 and the water temperature in the high-temperature-side cooling water circuit 31.

In the low-temperature-side cooling water circuit 32, a valve 65 is provided at the inlet side of the second heat exchanger 46, and a bypass channel 66 that bypasses the second heat exchanger 46 is provided. By opening/closing the valve 65, the flow of the cooling water flowing through the second heat exchanger 46 in the low-temperature-side cooling water circuit 32 is controlled.

In the third embodiment, the cooling device 1 includes a third water temperature gauge 61 that detects the water temperature of the cooling water flowing into the second heat exchanger 46 from the high-temperature-side cooling water circuit 31 and a fourth water temperature gauge 62 that detects the water temperature of the cooling water flowing into the second heat exchanger 46 from the low-temperature-side cooling water circuit 32.

As with the above-mentioned second embodiment, the controller 60 performs opening/closing control of the valve 65 on the basis of a water temperature TwH3 in the high-temperature-side cooling water circuit 31 detected by the third water temperature gauge 61 and a water temperature TwL4 in the low-temperature-side cooling water circuit 32 detected by the fourth water temperature gauge 62.

For example, when the water temperature TwL4 of the cooling water flowing into the second heat exchanger 46 in the low-temperature-side cooling water circuit 32 is lower than the water temperature TwH3 in the high-temperature-side cooling water circuit 31, the valve 65 is switched such that the heat exchange is performed at the second heat exchanger 46. With such a configuration, because it is possible to increase the cooling water temperature in the low-temperature-side cooling water circuit 32, the temperature of the cooling water flowing into the engine 10 is not decreased, and therefore, the delay in the warming up of the engine is improved. Because it is possible to decrease the cooling water temperature in the high-temperature-side cooling water circuit 31, the exhaust gas temperature can be further decreased in the high-temperature-side first EGR cooler 21.

On the other hand, when the water temperature TwH3 in the high-temperature-side cooling water circuit 31 is lower than the water temperature TwL4 of the cooling water flowing into the second heat exchanger 46 in the low-temperature-side cooling water circuit 32, the valve 65 is switched such that the heat exchange is not performed at the second heat exchanger 46. With such a configuration, because the heat exchange is not performed with the cooling water whose temperature is lower than that of the cooling water in the low-temperature-side cooling water circuit 32, the temperature of the cooling water flowing into the engine 10 is not decreased, and therefore, the delay in the warming up of the engine is improved. Because the heat exchange is not performed with the cooling water temperature in the high-temperature-side cooling water circuit 31 whose temperature is higher than the cooling water, it is possible to cool the exhaust gas further more in the high-temperature-side first EGR cooler 21.

As described above, in the third embodiment, after the temperature of the supercharged intake air is decreased as with the first and second embodiments, and in addition, after the exhaust gas temperature is decreased at the high-temperature-side first EGR cooler 21, it is possible to decrease the exhaust gas temperature at the low-temperature-side third EGR cooler 24 with the cooling water in the low-temperature-side cooling water circuit 32. Even with such a configuration, because it is possible to prevent decrease in the temperature of the cooling water that circulates through the high-temperature-side cooling water circuit 31 and the low-temperature-side cooling water circuit 32 and returns to the engine 10, it is possible to improve the delay in the warming up of the engine 10 even when the engine 10 is cold-started.

Embodiments of this invention were described above, but the above embodiments are merely examples of applications of this invention, and the technical scope of this invention is not limited to the specific constitutions of the above embodiments.

This application claims priority based on Japanese Patent Application No. 2013-036644 filed with the Japan Patent Office on Feb. 27, 2013, the entire contents of which are incorporated into this specification. 

1. An intake-air cooling device for an engine that is provided on the engine including a cooling water circuit through which cooling water for the engine flows and a supercharger that supercharges intake air for the engine, comprising a first intake-air cooling device, a second intake-air cooling device, and a heat exchanger, wherein: the cooling water circuit has a first cooling water circuit and a second cooling water circuit; the first intake-air cooling device is configured such that the intake air is cooled with the cooling water in the first cooling water circuit, and the second intake-air cooling device is configured such that, with the cooling water in the second cooling water circuit, the intake air that has been cooled by the first intake-air cooling device is further cooled; the first cooling water circuit is configured such that the cooling water that has come out from the engine passes through the first intake-air cooling device and returns to the engine again; the second cooling water circuit is configured such that the cooling water that has come out from the engine passes through the second intake-air cooling device and returns to the engine again; and the heat exchanger is configured to perform heat exchange between the cooling water that has come out from the engine and flowing to the first intake-air cooling device and the cooling water that has come out from the second intake-air cooling device and that is flowing to the engine.
 2. The intake-air cooling device for an engine according to claim 1, wherein the first cooling water circuit includes a first radiator for cooling the cooling water and the second cooling water circuit includes a second radiator for cooling the cooling water flowing through the second cooling water circuit, and the cooling water that has come out from the engine and that has been cooled by the second radiator flows through the second intake-air cooling device.
 3. The intake-air cooling device for an engine according to claim 1, wherein the cooling water in the first cooling water circuit that has come out from the first intake-air cooling device and the cooling water in the second cooling water circuit that has come out from the heat exchanger are combined and returned to the engine.
 4. The intake-air cooling device for an engine according to claim 1, further comprising: a first water temperature detector that detects water temperature of the cooling water in the first cooling water circuit entering the heat exchanger; a second water temperature detector that detects water temperature of the cooling water in the second cooling water circuit entering the heat exchanger; a bypass flow path that bypasses the cooling water in the second cooling water circuit such that the cooling water does not flow through the heat exchanger; a valve that controls whether the cooling water in the second cooling water circuit is allowed to flow through the bypass flow path; and a control device that is configured to control operation of the valve, wherein the control device performs: a control of the valve such that, when the water temperature of the cooling water in the second cooling water circuit is higher than the water temperature of the cooling water in the first cooling water circuit, the cooling water in the second cooling water circuit is allowed to flow through the heat exchanger; and a control of the valve such that, when the water temperature of the cooling water in the first cooling water circuit is higher than the water temperature of the cooling water in the second cooling water circuit, the cooling water in the second cooling water circuit is allowed to bypass the heat exchanger without flowing therethrough.
 5. A method of cooling intake-air for an engine including a cooling water circuit through which cooling water for the engine flows and a supercharger that supercharges the intake air for the engine, wherein the cooling water circuit has a first cooling water circuit and a second cooling water circuit; a first intake-air cooling device that is configured such that the intake air is cooled with the cooling water in the first cooling water circuit, and a second intake-air cooling device that is configured such that, with the cooling water in the second cooling water circuit, the intake air that has been cooled by the first intake-air cooling device is further cooled are provided; the first cooling water circuit is configured such that the cooling water that has come out from the engine passes through the first intake-air cooling device and returns to the engine again; the second cooling water circuit is configured such that the cooling water that has come out from the engine passes through the second intake-air cooling device and returns to the engine again; a heat exchanger that is configured to perform heat exchange between the cooling water that has come out from the engine and flowing to the first intake-air cooling device and the cooling water that has come out from the second intake-air cooling device and that is flowing to the engine is provided; a first water temperature detector that detects water temperature of the cooling water in the first cooling water circuit entering the heat exchanger is provided; a second water temperature detector that detects water temperature of the cooling water in the second cooling water circuit entering the heat exchanger is provided; and a bypass flow path that bypasses the cooling water in the second cooling water circuit such that the cooling water does not flow through the heat exchanger is provided; and a valve that controls whether the cooling water in the second cooling water circuit is allowed to flow through the bypass flow path is provided; the method comprising: controlling the valve such that, when the water temperature of the cooling water in the second cooling water circuit is higher than the water temperature of the cooling water in the first cooling water circuit, the cooling water in the second cooling water circuit is allowed to flow through the heat exchanger; and controlling the valve such that, when the water temperature of the cooling water in the first cooling water circuit is higher than the water temperature of the cooling water in the second cooling water circuit, the cooling water in the second cooling water circuit is allowed to bypass the heat exchanger without flowing therethrough. 